Almost, it's Proxima Nova Soft, a similar sans-serif. Gotham Rounded is not available as a webfont because Hoefler & Frere-Jones haven't come out with a webfont service yet, so Proxima Nova (and its Soft variant) is very popular as a "replacement" around the web (although don't get me wrong, the font is just as beautiful as Gotham on its own).
You should put a Gift option on the site (as a nudge nudge), I know plenty o designer friends that will find your book helpful and at 3/6 bucks, I'll gladly gift a few!
TAKE MY MONEY.
Seriously though, thanks for providing me something to read on the subway.
PS: Is that Gotham Rounded?